This application relates to a control for a refrigerant system wherein pulse width modulation technologies are utilized to provide further control over the system operation and wherein a control over a suction pressure is desired when the pulse width modulating valve is in a closed position.
Various operational features are known for providing adjustments in refrigerant system capacity. One approach, which has been utilized in the prior art to change the capacity of a refrigerant system, is the use of pulse width modulation to control valves such as a shutoff valve on a compressor suction line. By rapidly cycling these valves utilizing pulse width modulation techniques, additional capacity control is provided. A system with a pulse width modulating valve will perform most efficiently (in other words, the compressor will consume less power for the same delivered capacity) if the pressure inside the compressor shell within the suction region is reduced to the lowest possible value.
One challenge with regard to utilizing pulse width modulation control for a suction valve and reducing the pressure to the lowest possible value for the purpose of increasing operational efficiency is that, when the suction valve is closed to reduce capacity, the pressure within a compressor shell may drop to be effectively near complete vacuum. It is undesirable to have a pressure below a minimum predetermined pressure in the compressor shell, as this may sometimes lead to an undesirable condition called “corona discharge.” Also, the motor can overheat causing the discharge temperature to be above the specified safe limit. A corona discharge can occur over the electrical terminals, and may lead to a deterioration of the terminals, and compressor failure. This minimum pressure is typically around 0.5 psia.
In the past, this problem has been addressed by having a leaky pulse width modulating valve (for example, the valve with a small opening in its stem) or the bypass loop has been installed around the pulse width modulating valve. The small opening through the leaky pulse width modulating valve or the small opening in the bypass line was sized to maintain the pressure in the compressor suction compartment above a certain value. However, due to changing operating conditions, the size of this opening needed to be designed to assure that the pressure would not drop below a certain level for all operating conditions. Since the pressure needed to be above a minimum value for all operating conditions, this led to a situation where for some operating condition, the compressor was operating at suction pressures that were well above the minimum acceptable limit. This in turn led to lower system efficiency, because now the pressure was too high to assure the efficient operation at such “off-design” conditions. Thus, the need exists to adjust the suction pressure to be just above the minimum acceptable level for all operating conditions. In other words, in the prior art, the pressure could not be controlled to the minimum acceptable level for all operating conditions, causing the pressure to be higher than desired for some of the operating conditions, that led to lower efficiency in these instances. Thus, it would be desirable to maintain the pressure at the lowest possible level for all the operating conditions, when a suction valve is pulse width modulated to a closed position.